1. Field of the Invention
The present invention relates in general to railway car trucks and, in particular, to railway truck air spring suspensions. More specifically, but without restriction to the particular use which is shown and described, this invention relates to railway truck leveling valves which better control the overall height of railcars.
2. Description of the Related Art
It is known that in passenger railway cars, especially for high-speed trains, the use of air springs absorb the track shocks and vibrations of the railway trucks such that the vibrations are not greatly perceptible in the railway car body. It is also known that air spring suspensions on railway trucks attempt to maintain an essentially constant railway car floor height relative to the railroad track and relative to the height of a station platform. The air spring suspensions attempt to maintain this constant floor height over an entire load range, that is, from an empty railway car to a full car. A constant railway floor height, for example, facilitates safer loading and unloading of passengers, especially disabled persons.
U.S. Pat. No. 5,588,367 issued to Scheffel generally teaches railway vehicle suspensions and is incorporated by reference. Typically, a passenger railway truck has both a primary and a secondary suspension system. The secondary suspension generally comprises air springs and provides the majority of the total equivalent spring deflection and is thus the dominant factor in determining passenger ride comfort. The primary suspension generally comprises elastomeric or steel springs and contributes to ride comfort to a lesser degree. The most vital function of the primary suspension is to provide adequate wheel load equalization so the train can safely traverse uneven track.
Conventionally, the air spring height of the secondary suspension is controlled by a pneumatic level regulating valve connected to the air spring. The air spring is disposed between the truck side frame and truck bolster. Air springs are located on each side of the railway truck and it is customary to employ a level regulating valve with each of the air springs. Thus, the air springs for each railway truck maintain constant height and control the railway car floor height above the railway truck frame. As the railway car is loaded or unloaded with passengers, the level regulating valve controls the air pressure in the air springs in accordance with the loading of the car body, such that, as the loading increases, the pressure in the air springs is increased. Similarly, as the loading decreases, air pressure in the air springs is discharged. The increase or decrease in air pressure maintains the constant car floor height above the truck frame. The level regulating valve associated with each air spring will operate independently of the other level regulating valves to admit and exhaust air pressure from its associated air spring. This independent level regulating compensates for uneven loading of the railway car laterally from the longitudinal center line of the railway car. This insures that the railcar door threshold is maintained somewhat level with the station platform thus facilitating safe entry and egress of passengers.
While adequately measuring and compensating for the height of the railway car above the truck frame, the level regulating valve ignores the smaller deflections occurring in the primary suspension resulting from variations in the weight and uneven weight distribution of the railway car. That is, while the secondary suspension compensates for the loading of the railway cars in maintaining a constant railway car floor height above the level of the truck frame, the conventional suspension arrangements do not account for primary suspension deflections below the level of the truck frame. This is undesirable especially for passenger railway cars because vertical spring deflections and tilting of the car body leads to an uneven car floor height in relation to the railroad station platform. Under such conditions, it can be hazardous when passengers board and deboard the railway car, especially passengers with disabilities.